Noise reduction system



w. R. KOCH NOISE REDUCTION SYSTEM July 25, 1939.

Filed May 28, 1936 Time Snnentor Gttomeg Patented July 25, 1939 UNITEDSTATES 2,166,995 NOISE REDUCTION SYSTEM Winfield R. Koch, Merchantville,N. 1., assignor to Radio Corporation of America, a corporation ofDelaware Application May 28, 1936, Serial No. 82,204

Claims.

The present invention relates to a noise reduction system for radioreceiving apparatus and the like, which operates to suppress the signaloutput of the receiving system during a pulse of noise voltage to whichthe receiving system may be subjected in operation.

It is an object of the present invention to provide a noise reductionsystem for radio receiving apparatus and the like which operates tobreak up the noise impulse at a high frequency rate whereby it isprevented from passing through a low-pass or audio-frequency-passfilter.

It is a further object of the present invention to provide an improvednoise reduction system for a superheterodyne receiver, which may beapplied to the second detector circuit, whereby a filter in the audiofrequency output circuit from the second detector may be utilized inconjunction with means for breaking up a noise impulse in the detectoroutput circuit, to reduce the amplitude of a noise impulse below thelevel of the signal amplitude in that circuit.

It is also a further object of the invention to provide a noise limitersystem which maybe applied effectively to a second or audio frequencydetector circuit of a superheterodyne receiver.

The invention will be better understood, however, from the followingdescription when considered in connection with the accompanying drawing,and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a noisereduction system for a radio receiver, embodying the invention, and

Fig. 2 is a series of curves illustrating the mode of operation of thesystem of Fig. 1..

Referring to Fig. 1, a modulated signal input circuit is indicated at 5which may be the output circuit of an intermediate frequency amplifier(not shown) for which the coupling transformer 6 serves as an outputcoupling device for the second or audio frequency detector I of asuperheterodyne receiving system, and an automatic volume control device8 for the system, both of which are diode rectifiers in the presentexample. The detector and automatic volume control devices are connectedto a tuned secondary circuit 9 in connection with the transformer 6,which circuit is responsive to the intermediate frequency of thereceiver. The automatic volume control device is provided with asuitable output resistor ID from which is taken automatic volume controlpotentials for controlling the receiver through the lead H, in anysuitable and well known manner.

The rectifier l is also provided with a diode output resistor 12, thediode circuit being completed from the tuned circuit 9 through a supplylead id to the rectifier l, and thence through the output resistor l2 toground I4 and returning through ground I5 to the tuned circuit 9. Theautomatic volume control diode output resistor I0 likewise is providedwith a ground return IE to the input circuit 9.

Audio frequency signals derived from the output resistor l2 of thedetector 1 are conveyed through a lead H to the control grid IQ of afirst stage audio frequency amplifier tube l9, having a cathode 26 andan output anode 2 I. The cathode is connected through a self-biasresistor 22 with the chassis or ground 23, the connection including abiasi battery or nt al sou ce having a pole. dic potential of the sourceed in o r the potential of the self-bias source 22. It will 20 be notedthat the polarity of the output resistor l2 is such that the detectordirect current output voltage developed across said resistor serves tooppose the potential of the sources 22 and 24 in the grid circuitcomprising the lead H, the resistor 12, the ground connections l4 and 23and the potential sources 24 and 22 between the grid 18 and the cathode20 of the amplifier device [9. The arrangement is such that as highlymodulated signals are applied to the second detector, the amplifierdevice I!) is caused to operate normally as an amplifier withoutdistortion.

The audio frequency output circuit for the detector, which isrepresented by the lead ll and ground, and which in the present exampleis connected with a first stage audio frequency amplifier l9 asdescribed, is further provided with a noise suppressor pentode amplifierdevice 25 and a diode rectifier device or limiter 26, both of which areconnected in shunt relation to the audio frequency circuit.

In the present example, the diode rectifier or limiter 26 is providedwith an anode 21, connected with the audio frequency circuit lead I1,and having a cathode 28 connected with ground 29 through bias potentialsupply means 36 for the diode, comprising a battery in the presentexample. For the purpose of permitting the diode 26 to limit signal ornoise peaks, a series resistor 3| is provided in the audio frequencycircuit I1 between the diode and the detector output resistor l2.

The arrangement is such that as signals having peak amplitudes greaterthan the potential of the'battery are received in the audio frequency 55circuit, the diode serves to rectify the positive peaks thereof, therebylimiting their amplitude.

In order that the limiting action may take place above the level of thedesired signal, the voltage of the source 30 may be adjusted to equalsubstantially 100% modulation. The polarity of the biasing battery issuch that the cathode 28 is at the predetermined positive potential withrespect to the anode 2'! sufficient to prevent rectification atamplitudes below 100% modulation.

The limiting action occurs when the diode rectifier or limiter 26 beginsto draw current from the detector output resistor l2 through thepotential drop producing series resistor 3| thereby preventing theapplication ofv excessively high noise potentials to the audio frequencyamplifier Referring to Fig. 2, the curve A shows an alternating currentsignal wave 32 which may be an audio frequency signal at any frequencysuch as 1000 cycles, on which occurs a noise impulse 33 greater than100% modulation and on which the diode limiter operates to produce aflattening of the top of the impulse wave as indicated by the curve B at34.

In order to break up the limited noise impulse into a series of shortimpulses, the noise reduction pentode 25 is employed as a choppinglimiter to introduce into the audio frequency system at the time of theimpulse, a high frequency impulse, preferably super-audible, and undercontrol of the noise impulse.

For this purpose, an electrode, preferably the screengrid 35, of thepentode, is connected to a source of high frequency oscillations 36, anda screen grid potential supply source 3! having a ground connection 38to the common return circuit for the system. The cathode 39 andsuppressor grid 40 are also connected with ground as indicated at 4|.

The super-audible high frequency impulses from the generator 36 areapplied to the audio frequency output circuit l138 through the pentodedevice 25, the anode 42 of which is connected with the audio frequencycircuit lead I1, thereby placing the space path of the pentode device 25in shunt across the audio frequency circuit in parallel with the diodelimiter 26.

The noise impulse transmitted through the audio frequency circuit isthereby broken into a series of impulses indicated at 43 on the curve Cof Fig. 2. This super-audible wave then representing and resulting fromthe noise impulse, is substantially removed from the audio frequencysystem by a suitable low pass filter 44 in the audio frequency circuitat any suitable point following the noise reduction and limitingcircuit. In the present example, the filter comprises a series chokecoil 45 and by-pass capacitors 46 to ground 4'! connected with theoutput circuit 48 for the anode 2| of the-first stage audio frequencyamplifier IS. The coupling is provided through a suitable anode outputresistor 49 and coupling capacitor 50. r

The filter 44 serves to remove the impulses by V filtering action toproduce the reduced noise peak indicated at in connection with the curveD of Fig. 2. The filtered audio frequency current is applied to anysuitable utilizing circuit through an output lead indicated at 52.

In order that the noise reduction chopping ..wave or signal may beapplied to the audio frequency. circm't only when a noise impulse isreceived, a control circuit for the noise reduction pentode is provided.This comprises a diode rectifier device 55 having an anode 56 and acathode 51 connected with an input circuit comprising an untunedcoupling winding 58 and having an output circuit including outputresistor-59 and completing a diode rectifier circuit. put resistoroperates with the polarity shown and receives energy through therectifier device from. the coupling winding which is inductively coupledwith the signal channel, for example, through a coupling indicated inthe drawing, with the secondary of the intermediate frequency outputtransformer 6. V

The negative terminal of the control diode output resistor 59 isconnected to ground 60 through a source of biasing potential 6|, thepositive terminal of which is connected with ground. The positiveterminal of the resistor 59 is connected through a bias supply lead 62with the control grid 63 of the pentode device 25 thereby operating toapply to the grid 63 a negative biasing potential from the source 6|through the control diode output resistor 59.

In the presence of signals, the additional potential in series with thegrid 63 and the cathode 39 is provided by the output resistor 59 of thecontrol diode which potential is in opposition to the normal biasingpotential and varies with the signal strength. The arrangement is suchthat the normal biasing potential provided by the source BI issuflicient to bias the noise reduction pentode substantially to cut-offand to maintain it in the cut-off range for signals below 100%modulation, whereby the oscillations from the source 36 are preventedfrom being applied to the audio frequency output circuit from thedetector until a strong noise impulse is received which permits thepentode to function.

From the foregoing description, it will be seen that a noise reductionsystem for a radio receiver in accordance with the invention operates tobreak up the noise impulse or voltage wave at a high frequency rate inthe audio frequency circuit following the signal rectifier or audiofrequency detector. After the resultant wave is passed through the lowpass filter, the output during the pulse will be very nearly zero. Thenoise impulse in the audio frequency circuit may be broken up bysuper-audible or high frequency oscillations and the high frequencycomponent of the noise impulse may then be filtered out by the low passfilter. Since this limiter and noise reduction circuit is included inconnection with the second or audio detector of a receiver, the low passfilter may be an audio frequency filterhaving a cut-off point above theaudio frequency range. The energy of the high frequency component orsuper-audible components of the noise impulse will then be materiallyreduced in the audio frequency output circuit.

It is desirable in order to provide a condition of operation wherein thearea of the impulse wave above and below the normal carrier level. maybe substantially the same, that the diode limiter The diode outshouldbeadjusted for operation at substantially double the normal outputvoltage produced by dicated in Fig. 2C.

The low pass filter should be arranged to pass substantially the same orsomewhat smaller, band of audio frequency signals as'will be produced inthe second detector by the interaction of the carrier and such sidebands as will be permitted to reach the second detector through theselectivity provided by the signal selecting and receiving circuits suchas the intermediate frequency circuit 569.

I claim as my invention:

1. The combination with a radio receiver having an audio frequencydetector subject to noise impulses and having an audio frequency signaloutput circuit through which said noise impulses and desired signals areconveyed from said detector, of means for breaking up in said outputcircuit said noise impulses at a relatively high and super-audiblefrequency, and means for reducing the audio frequency output during thenoise impulses to substantially zero comprising a signal and impulselimiter operative about a mean zero modulation axis and a low passfilter following said audio frequency output circuit.

2. In a superheterodyne radio receiving system, the combination with thesecond detector thereof, of an audio frequency signal output circuit forsaid detector, means connected with said circuit providing a signal peaklimiter for maintaining the amplitude of received noise impulses below apredetermined high level, an electric discharge amplifier coupled tosaid output circuit, means for applying oscillations to said circuitthrough said amplifier, and means for controlling said amplifier inresponse to received noise impulses to permit the amplifier to operateonly in response to impulses above a predetermined amplitude.

3. In a radio receiver,. the combination with signal receiving means, ofan audio frequency detector for received signals, an audio frequencyamplifier coupled thereto having a low pass filter in the output circuitthereof, a diode signal limiter device connected in circuit between saiddetector and said audio frequency amplifier to limit signals transmittedfrom said detector to said amplifier to predetermined peak values, andmeans for introducing super-audible oscillations into the audiofrequency circuit between said detector and said audio frequencyamplifier in response to noise impulses having peak values above saidpredetermined values.

4. In a radio receiver, the combination with modulated signal receivingmeans, of an audio frequency detector for received signals, an audiofrequency amplifier coupled thereto having a low pass filter in theoutput circuit thereof, a diode signal limiter device connected incircuit between said detector and said audio frequency amplifier tolimit signals transmitted from said detector to said amplifier, biasingmeans for the limiter device to prevent operation thereof on noise andsignal peaks below 100% modulation, means for introducing super-audibleoscillations into the audio frequency circuit between said detector andsaid audio frequency amplifier, and means connected with said last namedmeans for operating the same in response to noise impulses having peakvalues above 100% modulation.

5. A noise reduction system including in combination, a diode signalrectifier having an audio frequency output circuit, an audio frequencysignal limiter connected with said audio frequency output circuit, meansfor adjusting said limiter to operate in response to signal peaks havingamplitudes above a predetermined high audio signal peak, an electricdischarge device having its space path connected in parallel with saidaudio frequency circuit, means for introducing super-audibleoscillations into said audio frequency circuit through said electricdischarge device to interrupt a noise impulse and including an electrodein said device and an oscillation generating device connected therewith,said electric discharge device further having a gain control electrode,means for biasing said gain control electrode to prevent oscillationsfrom being applied to said audio frequency output circuit through saiddevice in the presence of signals of normal amplitude, means responsiveto, signal impulses of a predetermined high value for causing saidelectric discharge device to operate to transmit said oscillations tosaid audio frequency output circuit, and a low pass filter in connectionwith the audio frequency output circuit following said limiter forremoving the super-audible component of the noise impulses.

6. In a radio receiving system of the superheterodyne type, thecombination of a diode second detector having a diode output resistorthe negative terminal of which is connected with the receiving systemground, a diode rectifier device having a diode output resistor thenegative terminal of which is connected to said ground through anopposing source of biasing potential, means for applying receivedmodulated signals to said detector and rectifier, an audio frequencyoutput lead connected with the positive terminal of said detector outputresistor, an audio frequency amplifier device having a control gridconnected to said output lead, an audio frequency output circuit forsaid amplifier device, a low pass filter in said last named outputcircuit, a third diode rectifier device and a pentode electric dischargedevice having space paths connected in parallel across the audiofrequency output circuit of said detector between said output lead andsaid ground, a series limiting resistor in said output lead adjacent tothe detector output resistor and biasing potential supply means for saidthird diode rectifier whereby the latter operates as a signal limiterfor signal peaks above a predetermined amplitude, means for applyinghigh frequency oscillations to an electrode of said pentode device, andmeans for applying a controlling potential from said second dioderectifier output resistor to the control grid of said pentode devicewhereby it is rendered operative to apply said oscillations to theaudiofrequency circuit in the presence of noise impulse peaks above saidpredetermined amplitude.

7. A noise reduction system including in combination, a diode signalrectifier having an audio frequency output circuit, an audio frequencysignal limiter connected with said audio frequency output circuit, meansfor adjusting said limiter to operate in response to signal peaks havingamplitudes above a predetermined high value, an electric dischargedevice having its space path connected in parallel with said audiofrequency circuit, means for introducing super-audible oscillations intosaid audio frequency circuit through said electric discharge device tointerrupt a noise impulse, means responsive to signal impulses abovesaid predetermined high value for causing said electric discharge deviceto operate to transmit said oscillations to said audio frequency outputcircuit, and a low pass filter in connection with the audio frequencyoutput circuit following said limiter for removing the super-audiblecomponent of the noise impulses.

8. In a radio receiving system having an audio frequency detectorsubject to noise impulses and having an audio frequency signal outputcircuit through which said noise impulses and desired signals areconveyed from said detector, the combination of a diode rectifierconnected across said audio frequency signal output circuit to provide asignal limiter for said circuit, means connected with said rectifier forcausing the limiting action to take place above a desired signal level,means providing a source of super-audible oscillations, a noisereduction device through which said oscillations are applied to saidsignal output circuit, said device having a control electrode, means forderiving a control potential for said' electrode which varies inaccordance with the strength of received signals, and means providing alow pass filter following said audio frequency signal output circuit forremoving from the signal output the super-audible component of aninterrupted noise impulse.

9. In a radio receiving system, means providing noise reductioncomprising an audio frequency detector having an output circuit, meansconnected with said output circuit for interrupting therein a noiseimpulse in a series of superaudible Waves, means connected with saidoutput circuit providing a low pass filter therein for removing saidsuper-audible Waves, andsaid first named means including a tube havingits space path connected across said output circuit and having twocontrol grids, an oscillator connected to one of said grids for applyingthereto a superaudible wave and a signal rectifier having an outputcircuit connected with the other of said grids for applying therectified signal component thereto above a predetermined amplitude.

10. In a radio receiving system, means providing noise reductioncomprising an audio frequency detector having an output circuit,animpulse limiter connected. with said output circuit, means connectedwith said output circuit for interrupting therein a limited noiseimpulse in a series of super-audible Waves, and means connected withsaid output circuit providing a low pass filter therein for removingsaid super-audible waves.

WINFIELD R. KOCH.

